1. Field of the Invention
The present invention relates to a method of enhancing image sharpness when scanning an imaging plane with a scanning-type image recorder, such as a process color scanner, for use in reproducing an image on the imaging plane.
2. Description of Background Art
The technique of visually enhancing sharpness of an image by amplifying density difference between contours of respective patterns in the image for use in reproducing the image with a process color scanner or the like is well known in the art. In such a conventional technique, an unsharp signal is obtained by taking the weighted average of respective image signals in a plurality of pixels arranged in the form of a matrix. A sharpness enhancement signal is generated from the unsharp signal and an image signal (sharp signal) of a central pixel of the matrix. The sharpness enhancement signal is added to an original image signal. A signal obtained by such addition is supplied to an image recorder as an image signal expressing an image in which sharpness is enhanced.
In the aforementioned technique, degrees of enhancement for respective color component signals of yellow (Y), magenta (M) and cyan (C) are equal to each other. If a part of the color-component signals is saturated in its signal value through enhancement, the following problem is caused:
When the boundary between a white region and a red region on the image is enhanced, for example, a yellow signal and a magenta signal expressing the red region are saturated at a certain saturation level. However, a cyan signal, being below the saturation level, is increased in the boundary, whereby an undesirable black contour appears in the side of the red region.
Thus, according to the prior art, a block contour undesirably appears around a region which has been subjected to contour enhancement. In order to prevent such a problem, there has been proposed a countermeasure of performing no sharpness enhancement with respect to a specific color phase. For example, the technique of disabling sharpness enhancement with respect to red is called a cut-red method. However, since a pattern of the specific color phase is not subjected to sharpness enhancement, apparent quality of a recorded image is lowered when the pattern includes the specific color component as a dominant color component.
Another problem is that, when a color component signal is saturated or enhanced, original image signals are varied in polarity with colors during sharpness enhancement of an original image, lines of different color phases appear on portions subjected to sharpness enhancement. Such a phenomenon is easily caused in a boundary portion between a gray (intermediate color) region and a region of a pure color having high chroma. An unnatural color pseudo-boundary, which is not present in the original, appears on a reproduced image. Thus, it is necessary to suppress the appearance of such a color pseudo-boundary.